The exhaust gas discharged from internal combustion engines such as diesel engines contains particulate matter (hereafter also referred to as PM) such as soot and the like, and in recent years, the harm caused to people and the environment by this PM has become an issue. Furthermore, because the exhaust gas also contains harmful gas components such as CO, HC and NOx, the effect of these harmful gas components on people and the environment is also a cause for concern.
Accordingly, in order to trap the PM in the exhaust gas and purge the harmful gas components, an exhaust gas cleaning device is used.
This type of exhaust gas cleaning device is prepared using a honeycomb filter composed of a material such as a ceramic. The exhaust gas can be cleaned by passing the exhaust gas through the honeycomb filter.
In a honeycomb filter used in an exhaust gas cleaning device to trap the PM in an exhaust gas, a multitude of cells separated by cell walls are disposed in parallel in the longitudinal direction, and each cell is sealed at one end section. Consequently, the exhaust gas that flows into a given cell necessarily passes through a cell wall that separates the cells, and then flows out of another cell. In other words, if this type of honeycomb filter is provided in an exhaust gas cleaning device, then the PM contained in the exhaust gas is captured by the cell walls as it passes through the honeycomb filter. Accordingly, the cell walls of the honeycomb filter function as a filter that cleans the exhaust gas.
In the initial stage of PM trapping by the honeycomb filter, the PM infiltrates the fine pores in the cell walls and is trapped inside the cell walls, generating a “depth filtration” state in which the fine pores in the cell walls are blocked. In this depth filtration state, PM continues to accumulate inside (the fine pores in) the cell walls. As a result, a problem can occur in that immediately after the trapping of PM begins, the effective porosity of the cell walls is reduced, causing the pressure loss to suddenly increase.
Patent Document 1 discloses a honeycomb filter in which a composite region is formed by depositing particles on the surface layer portions of the cell walls that constitute the honeycomb filter.